The present invention relates to a system for controlling an electromagnetic clutch disposed between the crankshaft of an engine and an infinitely variable transmission of a motor vehicle.
A clutch torque control system for the starting of a vehicle is, for example, disclosed in Japanese Patent Laid Open 57-15024. In the system, the clutch current flowing in a coil of an electromagnetic clutch exponentially increases with respect to an increase of the speed of an engine for starting the vehicle. When vehicle speed exceeds a predetermined speed (for example 20 km/h), lock-up current flows through the coil, so that the clutch is entirely engaged. At deceleration of the vehicle, when the accelerator pedal of the vehicle is released and the vehicle speed decreases below the predetermined speed, the clutch current is cut off to disengage the clutch in response to operations of an accelerator switch and a vehicle speed switch.
In such a control system, if the accelerator pedal of the vehicle is depressed to close the accelerator switch at a low vehicle speed below the predetermined speed, the clutch current dependent on the engine speed passes through the coil, so that the clutch is partially engaged.
On the other hand, an infinitely variable belt-drive transmission combined with such an electromagnetic clutch is so designed as to increase the reduction ratio at a low vehicle speed below the predetermined speed. If the vehicle is accelerated and decelerated at the low vehicle speed where the transmission ratio is at large reduction ratios, the vehicle is greatly accelerated and decelerated due to the large reduction ratios. Such a great variation of the vehicle speed extremely decreases the driveability of the vehicle.